A conventional hard disk drive (HDD) employs a method that uses a magnetic head for writing magnetization information with a magnetic field generated from a coil.
Now, hard disk drives are demanded further to cope with higher density recording and their magnetic head tip parts are required to be fabricated more finely to meet the refinement of the recording domain being progressed in accordance with the requirements of such higher density recording. However, it is estimated that the magnetic field intensity to be generated from a magnetic head is lowered along with the refinement of the magnetic head tip part due to an influence of antimagnetic components generated from the magnetic head tip part itself.
On the other hand, to keep the magnetization of the recording domain stably, a level of about KuV/kBT>50 is required. Here, Ku denotes a magnetic anisotropy constant, V denotes a magnetization cubic volume, kB denotes a Boltzmann's constant, and T denotes a temperature. If V becomes minuter along with higher density recording, the coercivity of the material having a larger Ku value is required to overcome the thermal instability in the written magnetizing direction. A magnetic field for writing is required to be increased more than the conventional one to write a magnetizing direction in this material having high coercivity.
Thus the conventional method that uses a magnetic head for writing magnetization information in high density recording comes to its limit and now, users are in need of a new writing method that can take the place of the conventional method.
For example, if the recording density is over 1Tb/inch2, it is considered that the recording medium comes to require coercivity over 10 K0e.
To cope with such a demand, a thermally assisted writing method is proposed. This method heats the object recording medium locally with use of a laser beam to lower the coercivity only in the object region of writing, thereby enabling the writing with a magnetic field to be generated from the magnetic head. This technique can reduce the writing magnetic field, so that it is considered to be a promising technique for writing magnetization information, employable in high density recording. However, because the object region is heated with a focused laser beam, the refinement in that region becomes difficult and this has been a problem.
A method that uses no magnetic field is proposed by, for example, J. Slonczewski, J. Mag. Mag. Mater, 159, L1 (1966). The method employs a spin injection magnetization inversion technique. According to this method, spin polarized electrons are injected into an object magnetic medium to invert the magnetizing direction, thereby enabling writing in the medium. In the case of this method, however, the write current threshold value is as high as about 106 A/cm2. In addition, the medium requires wiring for taking in a current while it is in contact. Otherwise, the medium cannot take in a current enough. This is why the method is not suitable so much for writing on a non-wiring ultra-high density recording medium such as hard disk drives.
There is another proposed writing method that uses an electric field to control magnetization. For example, Mattsonet et al, Phys. Rev. Lett. 71, 185 (1993) discloses such a technique for controlling an exchange interaction to occur between ferromagnetic media by controlling a carrier density in an object semiconductor layer with an electric field in an in-stack structure composed of a ferromagnetic metal layer, a semiconductor layer, and a ferromagnetic metal layer.
Chun-Yoel Youi et al., J. Appl. Phys., 87, 5215 (2000) also discloses a technique for controlling the exchange interaction to occur between ferromagnetic materials by providing an insulation layer in a three-layer structure composed of a ferromagnetic metal layer, a nonmagnetic metal layer, and a ferromagnetic metal layer and applying a voltage between ferromagnetic metal layers.
Furthermore, JP-A No. 2001-196661 discloses a technique for providing a semiconductor layer outside a three-layer structure composed of a ferromagnetic metal layer, a nonmagnetic metal layer, and a ferromagnetic metal layer and controlling the exchange interaction to occur between ferromagnetic materials by controlling both the width and the height of a Schottky barrier to be generated at the phase boundary between a ferromagnetic metal layer and a semiconductor with an electric field.
JP-A No. 2004-342183 also discloses a method and an apparatus for applying a local electric field to a recording medium having a three-layer structure composed of a ferromagnetic metal layer, a nonmagnetic metal layer, and a ferromagnetic metal layer with use of a metal probe, thereby controlling magnetization.
Furthermore, the JP-A No. 2006-65927 discloses a method and an apparatus for applying a local electric field and an assist magnetic field to an object recording medium having a three-layer structure composed of a ferromagnetic metal layer, a nonmagnetic metal layer, and a ferromagnetic metal layer with use of a metal probe, thereby controlling magnetization. In JP-A No. 2006-65927, the polarity of the voltage applied to the metallic probe is changed from positive V0 to negative −V or from negative −V to positive V0, then an assist magnetic field is applied to the object medium in a magnetizing direction for writing, thereby inverting the magnetizing direction. In other words, both the voltage and the magnetic field are inverted.
Those magnetization control techniques that use an electric field respectively can improve recording density and reduce power consumption. Thus they can be considered to be promising techniques.